Controller for liquid level or the like



Feb. 24, 1953 D. K. MclLvAlNE ETAL 2,629,826

CONTROLLER FOR LIQUID LEVEL OR THE LIKE 2 SHEETS-SHEET l Filed NO v. l2,1946 LEVfL Feb. 24, 1953 4D. K. MclLvAlNE ETAL 2,629,826

CONTROLLER FOR LIQUID LEVEL OR THE LIKE l Filed NOV. l2, 1946 2 SHEETS-SHEET 2 34 R5 [73mg iii nlm-v 'NIVEA/rows:

WML/A P MYERS ,Je

ArTaAn/EHS.

Patented Feb. 24, 1953 CONTROLLER FOR LIQUID LEVEL on THE LIKE DouglasK. Mcllvaine, Des Plaines, lll., and William l. Myers, Jr., WebsterGroves, Mo., as sgnors to Alco Valve Company, University City; Mo., acorporation of Missouri Application November 12, 1946, Serial No.709,134

(Cl. Z50-27) l2 Claims.

The present invention relates to a controller for liquid level, or thelike. In' general, this mechanism includes a sensitive element oonnectedinto an electronic circuit, which sensitive element responds to changesin the given external situation, and translates these changes intochanges in the output of an electronic cir'- cuit. The presentdisclosure exemplifies the sensitive element as one responding tochanges in liquid level, the illustrations speciically showing a float,which iioat operates an armature that moves with respect to a magneticcore in a characterized manner and thereby changes the mpedance of acoil as a function of the elevation of the oat. The coil, in turn, isconnected into an electronic circuit in a particular manner, so that thechanges in grid potential on a tube in the circuit are functions of theposition of the noat. These changes in grid potential are reilected inthe output circuit of the tube, which output circuit may be employed tooperate a valve controlling liquid flow from a tank, or the like, theliquid level in which regulates the position of the float.v It will beunderstood that external means other than a iioat may he used to operatethe sensitive element, and means other than a valve may be ultimatelyoperated o of the output circuit.

It is an object of the invention to provide a characterized magneticsolenoid coil and armature combination for the purpose of varying theimpedance of an electrical circuit, wherein a given amount of movementof the armature may always produce the nal operation or inopera-` tionof the output circuit, regardless of where such increment of movement isdisposed in the over-al1 path oi movement of the armature.

It is a further object of the invention to provide a mechanism andelectrical circuit of the foregoing kind including an electronictube soarranged as to render the tube conductive* or non-conductive inaccordance with variationsin the position of the armature element,"Which mechanism is operated Without resorting to oscillatory, resonant,or bridge network arrangements. h Y

A further object of the invention is to provide a mechanism asaforesaid, together with means for setting the control point foroperation of the output circuit at any place Within the Working range ofthe armature,

A further object of the invention is to provide a control as aforesaidhaving means for varying the magnitude of the uniform increments ofarmature movement necessary to cause4 the n: un

output circuit to become operative or inoperative, and vice versa. Moreparticularly, it is an object to provide the aforesaid operation whereinthe magnitude of the increments of armature movement may be varied tocause an electronic tube to become conducting or non-conducting, andvice Versa.

A particular object is to provide a controller as aforesaid in whichthe' operation of the load switch or relay may be altered by meansremote from the sensitive or governing element. In particular, it is anobject to provide a control as aforesaid in which a control point of anoutput circuit from an electronic tube may be calibrated by one means,that may be an internal means Within the mechanism, and may be adjustedby another means, which can be arranged externally and separately fromthe remainder of the circuit. Specifically, it is an object of theinvention to provide an output controlled mechanism, operated as aresult of operation of the output side of an electronic mechanism of theforegoing type, which changes the control point of the electronic tube,so that it is different when the output circuit is operative from whatit is when the said output circuit is inoperative.

It is a further object to provide a control as aforesaid which gives amaximum freedom from the effects of supply voltage variation. Aparticular object is to provide an electrical control as aforesaidhaving a combination of resistors, capacitors and electronic tubesenergized from a common source and so connected to an electronic outputtube as to provide the maximum freedom from the eiiects of supplyvoltage variation.

In the drawings:`

Fig. 1 is a more or less diagrammatic view of the principal elements ofthis invention;

Fig, 2 is a vertical section throughthe sensitive element; and

Fig. 3 Yis a wiring diagram of the control mechanism. h

As shown in Fig. 1, there is a liquid level tank Hl. This tank issupplied with liquid through a pipe Il. Ther invention is here appliedto a means for maintaining the liquid level in the tank It Withinprescribed limits. To this end, there is a sensitive mechanism,generally indicated at l2 and comprising a noat device, as will appear,a control, generally indicated at I3, an indicator i4, and a valvelvvhich may be of the electromagnetic type.

The sensitive element comprises a` liquid container I8 connected by alower pipe I9 and an upper pipe 2U with the tank it. By thisarrangement, the container I8 receives liquid at the level of the tank.It will appear that the mechanism I2 will respond to the liquid level inthe tank. The liquid level in the container I8 may vary Within a rangedetermined by the depth of this container, and this range is the rangeof adjustment of the mechanism.

As shown in Fig. 2, the container I3 is bowlshaped and has a iitting 22at its bottom, by means of which the pipe I9 may be connected into it.It likewise has a fitting 23 adjacent its upper edge, by means of whichthe pipe 2i) may be connected into it. At its top, it has a flange 24integrally attached to it, as by welding, or the like. This receives acap member, generally indicated at 25 and having a depending portion 2tthat ts closely down into the top of the bowl I8. The cap member 25 isremovably secured to the bowl member by screws 2l that interengage between the ilanges thereof. A suitable gasket 28 affords a seal betweenthese members.

Within the bowl, there is a Wire-like frame 29 that furnishes a guidemeans for a float 30. and this guide means or framework 2B minii'nizesfriction of the float.

The float gives vertical movement to an arma ture 3l, which is hereshown as a cylindrical tube constructed of magnetic material. Thearmature 3l is guided in an enclosing tube 32 of non-mag netic material,which tube 32 is attached to fitting 33 that is threaded into the bottomwall of' the cap 35 and affords a seal. The top of the tube 32 isenclosed by a plug 34. the float chamber is in communication with theinterior of the tube 32, but is sealed away from an interior coilchamber formed within the cap member 25 and a top cover 3S that isscrewed onto the top of the cap member 25.

Within the coil chamber 35, there is a coil 40 that is wound about asleeve 4I and inserted over the tube 32. A washer disc 42 fits againstthe top of the coil assembly and is urged downwardly by a coil spring 43that abuts against the inner part of the top cover 35. The coil isprovided with two leads 45 and 46 that pass out through a suitableopening 41 in the side of the cap meniber 25, which opening is threadedto receive a conduit fitting, as desired.

The armature 3| and coil 4I] are characterized to produce uniformoperational sensitivity. That is to say, a given increment of verticalmovement of the armature will produce a given increment of change inimpedance in the coil 4t, regardless of what vertical position thearmature occupies at the start of such given increment of movement. Inthe present case, this is obtained by having a long coil and acylindrical armature, in which the armature is long enough so that itscritical movement takes place when the armature is below the top of thecoil.

In Fig. 3, the wiring hookup is shown. A transformer has a primary 53connected across power lines 51 and 53, which may be controlled by amaster switch 5S. This transformer has irst secondary coil et, a secondsecondary coil Ii I, and a third secondary coil 52. The secondary 80 hasa lead 65 extending to a junction point with a line 66. This line leadsthrough the terminals of the indicator I4, and thence to a junction withthe lead 45 from the coil and armature assembly. This entire coil andassembly may be generally designated as M. The other lead 46 of the coilis connected by a wire 61 to a junction point of By this means,

a resistor RI. The other end of this resistor is connected to a wirethat, in turn, is connected to a wire 53 that leads back to thesecondary. The indicator I4 is conventionally connected to receivecurrent variations through the foregoing circuit, which vary with theposition of the armature in the coil, as will appear. The indicatorthereby registers the position of the oat 33.

The wire 61, at the upper end of the resistor RI, is likewise connectedby a wire I2 to the cathode 'I3 or" a tube '14. This tube mayconveniently be a 13H6 type of tube. The cathode `i3 is associa-ted witha plate 'F6 that is connected by a wire 'I'I that leads to one end of aresistor R2, the other end of which is connected to the line 68 and oneend of' the resistor RI.

A capacitor I9 is connected across the resistor R2 between the wires lland 69. The wire Il leads also through a resistor R3, a resistor R4 to agrid 8I of a tube 82 that may be of the 2050 type. A capacitor 83 andresistor R5 are connected t0 ground and disposed in series into the line'il between the resistors R3 and R4.

Returning to the line 55 leading from the secn ondary lill, this line isconnected with a line 85 that leads to a second cathode 35 in the tube14. This cathode is associated with a second plate 37 lthat is connectedby a wire 8G that leads to a wire 8S entering a resistance group R6, RIand R8. The other end of this resistance group is connected to a wirethat leads baci; to the wire 69 at its junction with the wire 68, andthence back to the secondary EU. A filter capacitor 9I is connectedacross the resistance group R6, Rl and R8 between the wires 83 and Si).

An adjustable divider arm 93 is movable across the resistor Rl and isconnected to ground.

A cathode 94 of the second tube 82 is connected to ground. This tube hasa plate 95. This plate 95 is connected by a wire SG that leads through arelay coil 91, the other end of which is connected by a wire 98 to oneend of the secondary 62. The other end of this secondary is connected bya wire 99 to ground. A resistancecapacitance group including a capacitor|99 and a resistor R9 in series is connected across the coil 91.

The secondary 62 is likewise connected by a wire IU2 through a resistorRill and capacitor |533 to the line 45 at its junction with the line 65to furnish a feed-back connection.

The coil 91, which as is evident, is on the output side of the tube 82,operates a relay having actuated blades |01 and I08. In their normal ror released positions, the two blades ID3 and |08 make contact withblades I 20 and II5, respectively. When the coil 97 is adequatelyenergized. these blades IDT and IUS make contact with blades H29 andIll, respectively.

The blade |01 is adapted to close with a contact |09. This contact isformed on a switch blade that is connected by a wire I IU to thejunction of the wires 88 and B9, the latter of which is connected intothe resistance group R6, RI and R8 and the divider arm 93. The blade IDIis connected by a wire I I I to all of a group of` resistors RII, RIZ,RIS, RI4, RIS and RIS. These resistors have contacts at their other endsthat are adapted to be engaged by a grounded contact I I2 which may beclosed with the end of any chosen one of the resistance group. As willappear, this resistance group RII through RIB typifles a variableresistance which, as is obvious, may be a siigle variable divider of thetype indicated at 9 'The switch blade |08 is here show-n as single.-pole, double-throw. When released, it is engaged with a contact |I5 thatis contacted to a wire IIS. When energized, the bla-de |08 closes withthe contact ||1 that is connected to a wire IIB. The blade |08 is itselfconnected to a Wire IIS. These three wires are connected in conventionalfashion through thev cable to the valve I5. This Valve typiiies theactuated means with which the present mechanism may work.

The secondary 8| has one side grounded at 99. Its other side X isconnected directly with the ends, also marked X, of heaters for thethree laments. Each filament is grounded at its other end.

Referring back to Fig. l, the control box I3 contains the various partsof the described mechanism exclusive of the sensitive mechanism i2, thegauge or indicator I4, and the valve I5. The knob indicated at 93 is theadjustable divider connected into the resistance group R6, R1 and R8,and the knob i|2 is the lead |I2 connectable with the resistance groupR| through RIG. The valve I5 is operated toward closed position when thewires IIE and IIB are closed through contact of the blade |08 and thecontact I I 5. The valve is opened by breaking of the foregoing circuit,and connection of the wires I I9 and |I8 through engagement of the blade|08 with the contact I I1. The valve may typically be a magnetic valvehaving two coils, one for opening and one for closing; or it may beoperated by a reversible motor.

Operation With the noat in any particular' position, the coil 40 willhave a certain impedance. When the switch 59 is closed and the primary56 of the transformer is energized, potential will be applied across thesecondary 60. One lead 65 of this secondary extends through the line 66,the impedance M, the line 61 to one end of the resistor RI. There is apotential applied across the resistor Rl that produces current flow,this flow continuing through the lines 68 and 69 back to the secondary.

The potential drop across the resistor RI is appli-ed across onecathode-plate group of the tube 14. The circuit; passes through the,line l? to the cathode 13, to the plate 16 and the line 11, to theresistor R2, andv thence back to the line 68. This produces a rectiedcurrent oW through `the resistor R2 and results in a potential dropacross this resistor R2.`

The potential drop across the resistor R2 is likewise applied across thegrid-cathode 8|-94 of the tube 82, thence to ground and back through thedivider 93 to the balance of the resistor group R1 and R8 and the line90 to the other end of the resistor R2. This potential is smoothed outby the capacitor 19.

Another potential is applied to the grid 8|. This is derived from theother cathode-plate group of the tube 14. It will be remembered that thesecondary 60 is connected by the line 65 through the line 85 to thecathode 86 of this tube. The circuit is continued via the plate 81 andthe lines 88 and 89 into the resistance group R6, R1 and R8. It returnsby the lines 90 and 59 to the secondary. However, the divider 93 isconnected into the resistor R1, and this divider is connected throughground with the cathode 94 of the tube 82. The grid 8| of the tube 82 islikewise connected to the line 69. Hence, there is a rectified potentialderived from this circuit applied to the grid 8|., which poe' tentialconsists of the voltage drop across the ref mainder of the resistor R1and the resistor R8 between the divider 93 and the line 90. Thispotential is adjustable by Varying the position of the divider 93.

Both of the foregoing rectiiied potentials are supplied by the tube 14.In addition, there is an alternating potential applied to the grid 8| bythe feed-back circuit. The feed-back line |02 leads from the secondaryG2 through the resistor RIU and the condenser |03, and may returnthrough the coil 40 and the resistors RI and R2 to the grid. Thecondenser |03 regulates the phase of this potential relative to theconducting cathode-plate potential across the tube 82, and, as willappear, assures the ring of the tube 82 at the proper point in itscycle.

As already noted, the adjustment of the di'A vider arm` 93 regulates thepotential on the grid 8| of the tube 82. The potential on the grid 8| islikewise a function of the drop across R|. This latter, in turn, is afunction of the impedance M of the coil assembly, which, in turn, is afunction of the position of the float 30. Hence, a given adjustment ofthe divider 93 will determine what position of the float 30 in the coil4Q will produce the proper impedance condition to eiect the desired netpotential on the grid 8| to cause the tube 82 to. become conducting.

Assuming that the tube 82 has become conducting, the relay coil 91 willbe energized. This will cause the two blades |01 and |08 to contactblades.v |09 and |1, respectively. This will cause the electricallevel-restoring device previously mentioned to function in the requiredmanner. When the blade |01 contacts blade |09, it introduces a newfactor in the determination of the potential on the grid 8|, because itintroduces a suitable one of the resistance group RI I through RI 6 inparallel with the resistor R6 and the first part of the resistance R1 upto the divider 93, and thereby changes the potential drop across theremainder of the resistor R1 and the resistor R8. The particularposition of the arm ||2 with a chosen one of the resistance group RIIthrough RIS determines the amount of this change in potential drop. Bythis means, a diierent potential is applied to the grid 8| of the tube82 after this tube becomes conducting. As a result of this, the float30. must move to a diiierent position to render the tube non-conductingfrom the position at which it caused the tube to become conducting. Or,assuming a differential in the tube due to its inherent characteristics,this differential is variable by adjusting the knob 2 with respect tothe resistances RI I through RIE. By this m-eans, a variableIdifferential may be obtained. Furthermore, this differential isadjustable: externally of the control mechanism itself, and may bephysically widely separated therefrom, as desired.

It will be seen, therefore, that the arm 93 is adjustable to select theparticular-elevation of the float 30 at which the tube 82 becomesconducting and the relay 91 is energized; whereas the knob H2, may beadjusted to determine the position of the float 3,0 at which the tubethereafter becomes non-conducting and the relay coil 91 deenergized forthe operation of the valve.

A consideration of the Wiring diagram will show that the potentialapplied to the grid 3| consists of a rectiiied potential across a partof the resistance R1 and the resistance R8, a rectification voltage.across the/resistor R2., and a. feed.-

back alternating voltage derived from the transformer secondary B2,flowing through the resistor RII) and the capacitor ID3.

The rectification voltage across the resistor R2 is a unidirectionalvoltage opposite in polarity to the voltage across the resistor group R1and R8, plus an alternating voltage of which the magnitude and wave formare determined by the values of the resistor R2 and the capacitor 19.The capacitor 19 smooths or iilters that rectiiied potential supplied bythe cathode-plate combination 13 and 16.

rIhe function of the potential derived from the resistor R2 is to varythe grid potential in accordance with the position of the float, becausethe voltage drop across R2 varies as a function oi the position of thefloat. The function of the voltage drop across the resistance group R1and R8 is to afford a means to adjust the point at whichV the tube 82becomes conducting and the actuated means 91 may be operated. Thefunction of the resistance group RII through Rie is to alter thislast-named potential applied to the grid 8| after the actuated means 91has been operated, so that the point at which the tube will becomenon-conducting may be adjusted at will. The result of this latter is, of

course, to change the diiierential of operation of the valve.

The function of the potential applied to the grid 8| by the feed-backcircuit is to insure the delivery of proper energy to the coil 91. Thisenergy is a function of the point in the cycle between cathode and plateof the tube 82, at which it becomes conducting. Since this latterdetermined by the secondary 62, and since also the feed-back circuit isphased from the secondary 62, this feed-back potential has its phasecontrollably adjusted with respect to the conductivity phase through thetube 82. In the practical application of the apparatus, the potentialapplied to the grid 8| by the feed-back will be substantially 90 aheadof the cathode-plate potential applied to the tube and will insure thatthe tube will become conducting at the beginning of the half cyclethrough the cathode-plate circuit.

If it is desired to change the position of the lioat at which the valvewill be operated, the divider arm 93 will be moved. 1f this arm is movedin the direction to reduce the potential drop applied to the grid 8l bythis resistance group, then a smaller potential drop must be suppliedacross the resistor R2 in order to make the tube 82 conducting. Thiswill require the iioat 30 to rise further than it did before in order toproduce conduction through the tube 82. Movement of the arm 93 in theopposite direction produces the opposite effect.

The change of the arm II2 on the resistance group RII4 through RIBincreases the voltage drop across the remainder of R1 and R8 by avariable amount. As the arm II2 is displaced from RIB toward RII, thevoltage drop across R1 and R8 increases. This spreads the difierentialof operation of the valve, because it requires the float 30 to withdrawits armature further than before in order to reduce the grid potentialon the grid BI below that at which the tube 82 becomes conducting.

The capacitor 9| is a filtering capacitor used in its conventionalmanner, tending to even the potential across the resistance group R6, R1and R8. The resistors R3 and R4, along with the resistor R and capacitor83, are used for isolating purposes for the stabilization of the tube82. The resistor R8 and the capacitor IDI) are used to stabilize theaction of the relay coil 81 and reduce chattering thereof.

It will be seen that a control arrangement has been provided thatattains the objectives set forth. In particular, it may be seen thatgreat flexibility is obtained by this mechanism, without resort tooscillatory, resonant, or bridge network connections. Likewise, thearrangement is relatively free of variations in supply voltage.

What is claimed is:

1. A control mechanism including an electronic tube having a cathode,grid means, and a plate, means interconnecting the cathode and the gridexternally oi the tube, circuit means for applying an operatingpotential across the cathode and plate, a sensitive element adapted toproduce a varying potential drop in response to variations in apredetermined physical condition, alternating potential producing means,a circuit having the sensitive element therein and having a resistancetherein, the circuit being connected across the potential producingmeans, a circuit across the resistance including a rectier and a secondresistance, means connecting the second resistance between the gridmeans and cathode, an additional circuit across the potential producingmeans, including a rectifier and a third resistance in series, meansconnecting variabie amounts of said third resistance between the gridmeans and the cathode to apply a potential to the grid means of polarityopposite to that derived from the second resistance, Whereby, when thegrid potential derived from the third resistance is varied, thepotential derived from the second resistance must be varied to attain agiven net grid potential, capable of causing the tube to conduct, andmeans operated when the tube becomes conductive, to vary the potentialapplied between the grid means and the cathode.

2. A control mechanism including an electronic tube having a cathode,grid means, and a plate, means interconnecting the cathode and the gridexternally of the tube, circuit means including switch means, forapplying an operating potential across the cathode and plate, asensitive element adapted to produce a varying potential drop inresponse to variations in a predetermined physical condition,alternating potential producing means, a circuit having the sensitiveelement therein and having a resistance therein, the circuit beingconnected across the potential producing means, a circuit across theresistance including a rectiiier and a second resistance, meansconnecting the second resistance between the grid means and the cathode,an additional circuit across the potential producing means, including arectiier and a third resistance in series, means connecting variableamounts of said third resistance between the grid means and the cathodeto apply a potential to the grid means of polarity opposite to thatderived from the second resistance, whereby, when the grid potentialderived from the third resistance is varied, the potential derived fromthe second resistance must be varied to attain a given net gridpotential, capable of causing the tube to conduct suiiiciently to effectoperation of the switch means, and means operated when the tube becomesconductive, to vary the potential applied between the grid means and thecathode, comprising an additional resistance connected by operation o1 9the switch means -to change 'the potential drop across the thirdresistance.

3. A control comprising an electronic tube having a cathode, grid meansand a. plate, circuit means applying a potential across the cathode andplate and energized when the tube is conductive, the tube beingconductive only when the grid means carrie-s adequate bias, asensitiveelement comprising a float, an armature operated by the float, a coilassociated with the armature, the coil and armature being characterizedto produce changes in impedance of the coil with changes in position ofthe armature, a source of alternating potential, a iirst rectiner and arst resistor connected in series across the potential source, a secondresistor, the coil and second resistor being connected across thepotential source, a sec-ond rectifier and a third resistor connectedacross. the second resistor, the cathode being connected to a'selectable part of the first resistor and the grid being connected tothe circuit between the second rectier and third resistor, a feed-backcircuit including a fourth resistor and a capacq itor connected betweenthe plate of the tube and a point between the coil and its source ofpotential, a switch closed upon conduction of the tube, a fifth andvariable resistor, circuit means from between the rst rcctier and firstresistance through the switch andthe fifth resistor to the point ofconnection. between the rst rectiner and the first resistor.

4. A control mechanism including an electronic tube having a cathode,grid means and a plate, means applying an alternating operatingpotential across the cathode and plate including an output circuit, asensitive element automatically operable to produce a varying potentialdrop in response to changes in predetermined physical conditions, meansto apply a unidirectional potential between the grid means and thecathode that varies with the potential drop of the sensitive element,selectively variable potential drop producing means, means applying asecond unidirectional potential between the grid means and the cathodethat varies with the potential drop produced by said selectivelyvariable means, whereby variation of said selectively variable meanswill regulate the potential on the grid means and vary the potentialdrop required of the sensitive element to render the tube conductive, acommon source for said two potentials, said means applying saidpotentials between the grid and the cathode being connected with saidsource to cause l said potentials to be applied in opposite directionswhereby the resultant; grid potential from them is the differencebetween the two, said operating potential means comprising an outputbranch connected to an alternating potential source, and means applyingan alternating potential between the grid means and the cathode inaddition to said two unidirectional potentials, which alternating gridpotential has a predetermined phase relationship to the alterhatingoperating potential, said means applying the alternating grid potentialto the grid means comprising circuit branch connections connected fromthe same alternating source as the operating potential, and phasechanging means in one of said alternating potential branches to producea predetermined phase relationship between the two alternatingpotentials.

5. In a control mechanism, 'an electronic tube having a plate andcathode circuit, and grid means, the,4 tube-being conductive to energizethe plate and cathode circuit when a. predeter mined grid potential isapplied between the grid means and the cathode, and being rendered non.conductive when the grid potential is thereafter changed, a sensitiveelement movable in response to changes in physical conditions, circuitmeans to produce a net grid potential that varies with movement of thesensitive element and has a value that is a function of the positionthereof, whereby the tube may become conductive when the sensitiveelement reaches a predetermined position, and means to variT the gridpotential when the plate circuit is energzed, tc vary the position ofthe sensitive element at which the plate circuit becomes nonconducting.

6. In a control mechanism, an electronic tube having a plate and cathodecircuit, and grid means, the tube being conductive to energize the platecircuit when a predetermined grid potential is applied between the gridmeans and the cathode, and being rendered non-conductive when the gridpotential is thereafter changed,

` a sensitive element movable in response to changes in physicalconditions, circuit means to produce a net grid potential that varieswith movement of the sensitive element and has a value that is afunction of the position thereof, whereby the tube may become conductivewhen the sensitive element reaches a predetermined position, and meansto vary the grid potential when the plate circuit :'s energized, to varythe position of the sensitive element at which the plate circuit becomesnon-conducting, said last recited means to vary the grid potentialcomprising a circuit branch rendered elective upon conduction ofthe tubeto alter the voltage drop in the grid circuit means.

'7. A control mechanism including an electronic tube having a plate, acathode, and a grid connected with the Cathode through a circuitexternal of the tube; means for providing an alternating potentialacross the cathode and plate; a sensitive element for automaticallyproducing a range of variable potentials in response to variations in aselected physical condition; means for continuously applying a rstpotential between the grid and the cathode that varies in response tovariations in the potential produced by the sensitive element; means forcontinuously applying a second potential between the grid and thecathode opposite to the first potential including selectively variablemeans to produce a predetermined potential; and means comprising analternating potential circuit containing a phase shifter, the grid, andthe cathode, to cause the tube to become conducting near the beginningof the alternating l potential cycle applied across the cathode andplate when the net potential provided by the rst and second potentialattains a predetermined value.

8. A control mechanism including an electronic tube having a plate, acathode, and a grid connected with the cath-ode through a circuitexternal of the tube; a circuit external of the tube connecting thecathode and the plate; a sensitive element for automatically producing'a range of variable potentials in response to variations in a selectedphysical condition; means for continuously applying a first variablepotential between the grid and the cathode that varies in response tovariations in the potential produced by the sensitive element; means forcontinuously applying a second potential between the grid and thecathode opposite to the iirst potential; a relay in the plate-cathodecircuit; and circuit means including the relay contacts and anadjustable resistor, the circuit means being operated by actuation ofthe relay and interconnected with the means continuously applying thesecond potential to the grid and cathode, to automatically change themagnitude of the second potential.

9. A control mechanism, comprising an electronic tube having a plate, acathode, and a grid; a grid-cathode circuit external of the tube; asensitive element movable in response to changes in physical conditions;circuit means to the net grid potential, to change the position of thesensitive element required to change the tube back to the othercondition.

l0. A control mechanism, comprising an electronic tube having a plate, acathode, and a grid;

a grid-cathode circuit external of the tube; a Y

sensitive element movable in response to changes in physical conditions;circuit means to produce a net potential between the grid and thecathode that varies with movement of the sensitive element and has avalue that is a function of the position thereof, the tube changingbetween a conducting condition and a non-conducting condition when thesensitive element reaches a iirst position; and means responsive to thechange of the tube from one condition to the other condition caused bythe sensitive element attaining said first position, to change theposition of the sensitive element required to change the tube back tothe other condition.

l1. A control mechanism including an electronic tube having a cathode,grid means, and a plate; means interconnecting the cathode and the gridexternally of the tube; circuit means for applying an operatingpotential across the cathode and plate; a sensitive element adapted toproduce a varying potential drop in response to variations in apredetermined physical condition; alternating potential producing means;a circuit having the sensitive element therein and having a resistancetherein, the circuit being connected across the potential producingmeans; a circuit connected in parallel with the resistance and includinga rectifier and a second resistance; means connecting the secondresistance in parallel with the grid means and cathode; an additionalcircuit across the potential producing means, including a rectier and athird resistance in series; and means connecting variable amounts ofsaid third resistance in parallel with the grid means and the cathode toapply a potential to the grid means of polarity opposite to that derivedfrom the second resistance.

12. A control mechanism including an electronic tube having a cathode,grid means, and a plate; means interconnecting the cathode and the gridexternally of the tube; circuit means for applying an operatingpotential across the cathode and plate; a sensitive element adapted toproduce a varying potential drop in response to variations in apredetermined physical condition; alternating potential producing means;a circuit having the sensitive element therein and having a resi-stancetherein, the circuit being connected across the potential producingmeans; a circuit connected between the sensitive element and theresistance and including a rectifier and a second resistance; meansconnecting the second resistance between the grid means and cathode; anadditional circuit across the potential producing mean-s, including arectier and a third resistance in series; means connecting variableamounts of said third resistance between the grid means and the cathodeto apply a potential to the grid means of polarity opposite to thatderived from the second resistance; and means responsive to the changein tube current between conduction and nonconduction conditions forchanging the current through the third resistance so as to vary thepotential derived from it.

DOUGLAS K. MCILVAINE. WILLIAM P. MYERS, Jn.

REFERENCES CITED The following references are of record in the iile ofthis patent:

UNITED STATES PATENTS

